A “smart gel” is a material that gels in response to a specific physical property. For example, it may gel at a specific temperature or pressure. Although finding many industrial uses, our interest in smart gels lies in their uses in oil and gas exploration, and in particular as a sweep fluid to improve oil recovery from reservoirs.
The water injection method used in oil production is where water is injected back into the reservoir usually to increase pressure and thereby stimulate production. Water is injected for two reasons: 1. For pressure support of the reservoir (also known as voidage replacement). 2. To sweep or displace the oil from the reservoir, and push it towards an oil production well.
Normally only 30% of the oil in a reservoir can be extracted, but water injection increases that percentage (known as the recovery factor) and maintains the production rate of a reservoir over a longer period of time.
However, sweep recovery is limited by the so-called “thief zones,” whereby water preferentially travels through the more porous regions of the reservoirs, bypassing less porous zones. One means of further improving recover is to block thief zones with a polymer or other material, thus forcing water through the less porous regions.
Gels are often used in drilling applications, because these fluids can be optimized for each reservoir by controlling the gelation process. For example, U.S. Pat. No. 5,399,269 describes a composition and method to delay the gelation and increase gel strength, comprising: (1) a water dispersible first crosslinking compound selected from the group consisting of p-aminosalicylic acid, furfuryl alcohol, RArOC(O)R′ and HOArC(O)OR″ wherein Ar represents a phenyl group which can be substituted or non-substituted; R is a hydrogen or a carboxylic group; R′ is a C1-C6 alkyl; R″ is a hydrogen, a phenyl group, or a C1-C6 alkyl; and when R is a carboxylic group, R and C(O)OR″ can be at ortho, meta, or para position with respect to the OC(O)R′ group and the OH group, respectively; (2) a water dispersible second crosslinking compound selected from the group consisting of aldehydes and aldehyde-generating compounds; (3) a water soluble acrylamide-containing polymer; and (4) water.
U.S. Pat. No. 5,480,933, in contrast, describes a composition and method for accelerating the gelation, comprising: (1) an ammonium ion donor; (2) a water soluble acrylamide-containing polymer; (3) a first crosslinking component selected from the group consisting of aldehydes and aldehyde-generating compounds; (4) a second crosslinking component selected from the group consisting of an aromatic compound and an alcohol where the aromatic compound is selected from the group consisting of phenols and acids; and (4) water.
U.S. Pat. No. 5,423,380 describes a process for treating a subterranean formation by adding a crosslinking agent intermittently to a stream of gellable polymer. This method allows a reduction in the total quantity of crosslinking agents needed, a desirable end since many crosslinkers are expensive.
U.S. Pat. No. 6,454,003 et seq, in particular describes what might be called a “smart gel” since its properties change in response to particular stimuli. This patent describes an expandable cross linked polymeric microparticle having an average particle diameter of about 0.05 to 10 microns. The particle is highly crosslinked with two crosslinkers, one that is stable and a second that is labile. The excess crosslinking makes the initial dry particles quite small, allowing efficient propagation through the pores of a reservoir. On heating to reservoir temperature and/or at a predetermined pH or other stimuli, the reversible (labile) internal crosslinks break, allowing the particle to further expand by absorbing additional injection fluid, usually water.
The unique properties of this particle allows it to fill the high permeability zones—commonly called thief zones or streaks—and then be expanded so that the swollen particle blocks the thief zones and subsequent injections of fluid are forced to enter the remainder of the reservoir, more effectively sweeping the reservoir. However, the method is limited in practice because subsequent injections always remove some of the polymer, thus the thief zones become washed out and again present the problem of allowing the injection fluid to avoid entering the less porous zones.
The reason for the washout is not certain, but probably relates to several factors. First, most swellable polymers are also squeezable under pressure. Thus, when the reservoir pressure increases on further injection of fluid, the swollen particle are squeezed, losing fluid and shrinking in size, and thus allowing the particle to wash out of the thief zone. Further, our own research suggests that the swollen polymer is not in gel form, thus although viscous, is a liquid and can be washed out of the porous substrate.
WO2007126318 teaches the use of polymers prepared in an oil-in-oil emulsion. The patent describes the use of an oil soluble monomer polymerized in such emulsion and crosslinked with a permanent as well as a water-labile crosslinker. The resulting crosslinked particles are kept water-free and injected in an organic solvent into the formation, typically producing wells. Once the polymer comes in contact with formation water it swells due to hydrolysis of the labile bonds. The resulting swelled polymers should block water flow and possibly increase oil production.
However, the feasibility of producing such polymers as described in this patent is questionable. According to this patent, the monomers should be oil soluble and at least two oil-soluble crosslinkers are needed to form such particles in which one of these crosslinkers should be water-labile. However, many of the monomers listed are water-soluble and would most likely not dissolve in an oil-in-oil emulsion to carry out the polymerization reaction. Further, the process is only employed in producing wells, not in injection wells.
What is needed in the art is a more stable “smart gel” that is gel stabilized and less susceptible to loss of fluid or polymer under the conditions of use. In particular, a swellable polymer that is resistant to wash out by subsequent fluid injections is needed, but the polymers will have utility in any application where stable swellable smart gels are desired.